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Surveillance Systems

equipped with an airborne transponder. All ARTCCs’

radars in the conterminous U.S. and many airport

surveillance radars have the capability to interrogate

Mode C and display altitude information to the

controller from appropriately equipped aircraft.

However, there are a number of airport surveillance

radars that don’t have Mode C display capability and;

therefore, altitude information must be obtained from

the pilot.

(f) At some locations within the ATC en route

environment, secondary−radar−only (no primary

radar) gap filler radar systems are used to give lower

altitude radar coverage between two larger radar

systems, each of which provides both primary and

secondary radar coverage. In those geographical

areas served by secondary−radar only, aircraft

without transponders cannot be provided with radar

service. Additionally, transponder equipped aircraft

cannot be provided with radar advisories concerning

primary targets and weather.


Pilot/Controller Glossary Term− Radar.

(g) The controller’s ability to advise a pilot

flying on instruments or in visual conditions of the

aircraft’s proximity to another aircraft will be limited

if the unknown aircraft is not observed on radar, if no

flight plan information is available, or if the volume

of traffic and workload prevent issuing traffic

information. The controller’s first priority is given to

establishing vertical, lateral, or longitudinal separa-

tion between aircraft flying IFR under the control of


c. FAA radar units operate continuously at the

locations shown in the Chart Supplement U.S., and

their services are available to all pilots, both civil and

military. Contact the associated FAA control tower or

ARTCC on any frequency guarded for initial

instructions, or in an emergency, any FAA facility for

information on the nearest radar service.

4−5−2. Air Traffic Control Radar Beacon

System (ATCRBS)

a. The ATCRBS, sometimes referred to as

secondary surveillance radar, consists of three main


1. Interrogator. Primary radar relies on a

signal being transmitted from the radar antenna site

and for this signal to be reflected or “bounced back”

from an object (such as an aircraft). This reflected

signal is then displayed as a “target” on the

controller’s radarscope. In the ATCRBS, the

Interrogator, a ground based radar beacon transmit-

ter−receiver, scans in synchronism with the primary

radar and transmits discrete radio signals which

repetitiously request all transponders, on the mode

being used, to reply. The replies received are then

mixed with the primary returns and both are

displayed on the same radarscope.

2. Transponder. This airborne radar beacon

transmitter−receiver automatically receives the sig-

nals from the interrogator and selectively replies with

a specific pulse group (code) only to those

interrogations being received on the mode to which

it is set. These replies are independent of, and much

stronger than a primary radar return.

3. Radarscope. The radarscope used by the

controller displays returns from both the primary

radar system and the ATCRBS. These returns, called

targets, are what the controller refers to in the control

and separation of traffic.

b. The job of identifying and maintaining

identification of primary radar targets is a long and

tedious task for the controller. Some of the

advantages of ATCRBS over primary radar are:

1. Reinforcement of radar targets.
2. Rapid target identification.
3. Unique display of selected codes.

c. A part of the ATCRBS ground equipment is the

decoder. This equipment enables a controller to

assign discrete transponder codes to each aircraft

under his/her control. Normally only one code will be

assigned for the entire flight. Assignments are made

by the ARTCC computer on the basis of the National

Beacon Code Allocation Plan. The equipment is also

designed to receive Mode C altitude information

from the aircraft.


Refer to figures with explanatory legends for an illustration

of the target symbology depicted on radar scopes in the

NAS Stage A (en route), the ARTS III (terminal) Systems,

and other nonautomated (broadband) radar systems. (See

FIG 4−5−2 and FIG 4−5−3.)

d. It should be emphasized that aircraft transpond-

ers greatly improve the effectiveness of radar



AIM, Paragraph 4−1−20 , Transponder Operation